After countless bins, I have flashed the .bin file that was provided in here, using that, for some reason, allowed me to flash from web GUI without a problem. I also met one single person who had the same issue and couldn't revert to stock firmware, so the individual stayed on openWRT for the rest of the time, so I informed him of that as well in case he needs it in the future.
It could be a very rare issue because the person in the linked thread wasn't able to flash stock firmware either.
I still haven't found a way to configure my interfaces for internet access with DHCP, but I was able to revert to stock firmware. If I manage to find a way to fix the internet access or find someone to help me, I will update this post with instructions.